Overlapping walls: edited mesh doesn't recalculate array modifier results
The mirror modifier works by taking into account where the geometry is in relation to the object origin (the orange dot at the centre of the model). The array modifier that controls where the walls are depends on the chosen Empty, which is at the same place as the object origin.

All you have to do is select all the geometry in edit mode and move it on the 'Y' axis (green), away from the object origin (by pressing G, Y) and both the Mirror modifier and the Array modifier will update.